This is a copy of a paper that was presented at the
Fourth Conference on Local Networks, Minneapolis, 1979.
It was later republished (with permission) in
the North-Holland Publishing Companies Computer Networks 3 (1979) pp. 389-399.

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Components of a Network Operating System

Recent advances in hardware interconnection techniques for local networks
have highlighted the inadequacies in existing software interconnection
technology. Though the idea of using a pure message-passing operating
system to improve this situation is not a new one, in the environment
of a mature, high speed, local area network it is an idea whose time has come.
A new timesharing system being developed at the Lawrence Livermore Laboratory
is a pure message-passing system. In this system, all services that are
typically obtained directly by a system call (reading and writing files,
terminal I/O, creating and controlling other processes, etc.) are instead
obtained by communicating via messages with system service processes
(which may be local or remote). The motivation for the development of
this new system and some design and implementation features needed
for its efficient operation are discussed.

The basic job performed by an operating system is
multiplexing the physical resources available on its
system (Fig. l). By a variety of techniques such as
time slicing, spooling, paging, reservation, allocation,
etc. the operating system transforms the available
physical resources into logical resources that can be
utilized by the active processes running under it
(Fig. 2).

Figure 2 - Logical resources made available to a user process.

The interface between a process running under an
operating system and the world outside its memory
space is the "system call", a request for service from
the operating system. The usual approach taken in
operating system design has been to provide distinct
system calls to obtain service for each type of available
local resource (Fig. 3).

If a network becomes available, system calls for
network communication are added to the others
already supported by the operating system. Some
problems with this approach are the Dual Access and
Dual Service Dichotomies discussed below. It is
argued here that operating systems to be connected
to a network (particularly a high speed local area network)
should be based on a pure message-passing monitor (Fig. 4)

Figure 4 - Resource interface for a message-passing operating system.

The title of this paper has at least two interpretations
that are consistent with the intent of the
author:

If the term "Network Operating System" is
taken to refer to a collection of cooperating computer
systems working together to provide services
by multiplexing the hardware resources available
on a network, then the title "Components of a
Network Operating System" suggests a discussion
of the "Component" systems.

On the other hand, the
term "Network Operating System" can also be taken to
refer to a single machine monitor to which the adjective
"Network" is applied to indicate a design that facilitates
network integration. In this case the title "Components of
a Network Operating System" suggests a discussion of
the component pieces or modules that comprise such a single machine
operating system.

The basic approach taken here will be to describe
the components of a single machine operating system
being implemented at the Lawrence Livermore
Laboratory (LLL). The presentation will be largely
machine independent, however, and will include discussion
of the integration of the described system
into a network of similar and dissimilar systems.

LLL has a long history of pushing the state of the
art in high speed scientific processing to satisfy the
prodigious raw processing requirements of the many
physics simulation codes run at the laboratory. The
high speed, often few of a kind computing engines (For example, Univac-1, 1953,
Larc, Remington Rand, 1960, Stretch, IBM, 1961, 6600, CDC, 1964,
Star-100, CDC, 1974, Cray-1, Cray Research, 1978)
utilized at LLL are usually purchased before mature
operating system software is available for them [22].
The very early operating systems implemented at
LLL were quite simple and were usually coded in
assembly language. By the time of the CDC 6600
(1965), however, they were becoming more comp1ex
timesharing systems. By 1966 it was decided to
write the operating system for the 6600 in a higher
level language. This decision made it easier to transfer
that system (dubbed LTSS, Livermore Time Sharing
System) to new machines as they arrived: CDC 7600,
CDC Star-100, and the Cray-l.

Another important development at LLL that
began about the time of the first LTSS was networking.
It started with a local packet switched message network
for sharing terminals and a star type
local file transport network for sharing central file
storage (e.g. the trillion bit IBM photodigital storage
device). These early networks worked out so well that
they eventually multiplied to include a Computer
Hardcopy Output Recording network, a Remote Job
Entry Terminal network, a data acquisition network,
a tape farm, a high speed MASS storage network, and
others. The entire interconnected facility has retained
the name "Octopus" [12,
27] from its earliest days as
a star topology.

Recent developments in high speed local networking [5,
11,17]
are making it easier to flexibly connect
new high speed processors into a comprehensive support
network like Octopus. This very ease of hardware interconnection,
however, is forcing some rethinking of software interconnection issues to
ensure that the software interconnects as easily as the
hardware [26, 27].

Recently the network systems group at LLL has
started down a significant new fork in the LTSS
development path. The new version of LTSS is
different enough from the existing versions that it has been
variously called New LTSS or Network LTSS
(NLTSS). Many of the reasons for the new development
have little to do with networking. For example,
NLTSS shares resources with capabilities [4,
8, 10,
16, 20,
21, 28].
This allows it to combine the relatively ad
hoc sharing mechanisms of older LTSS versions into a
more general facility providing principal-of-least-priviledge
domain protection. It is only the lowest level
network related motivations for the NLTSS development,
however, that we will consider here.

When a processor is added to a mature high speed
local area network like Octopus, it needs very little in the
way of peripherals [27]. For example, when a Cray-1
computer was recently added to Octopus, it came
with only some disks, a console, and a high speed net-
work interface. All of the other peripherals (terminals,
mass storage, printers, film output, tape drives,
card readers, etc. etc.) are accessed through the network.
The operating system on a machine like this
faces two basic problems when it is connected to the
network:

A. How to make all the facilities available on the network
available to its processes, and

B. How to make all of the resources that it and its
processes supply available to processes out on the
network (as well as its own processes).

Typical third generation operating systems have
concerned themselves with supplying local processes
access to local resources. They do this via operating
system calls. There are system calls for reading and
writing files (access to the storage resource), running
processes (access to the processing resource), reading
and writing tapes (access to a typical peripheral
resource), etc. When networks came along, it was
natural to supply access to the network resources by
supporting system calls to send and receive data on
the network (Fig. 3).

Unfortunately, however, this approach is fraught with
difficulties for existing operating systems. Just
supporting general network communication is not at all
an easy task, especially for operating systems without
a flexible interprocess communication facility. In fact,
if flexible network communication system calls are added
to an operating system, they provide a de facto interprocess
communication mechanism (though usually with too much
overhead for effective local use).

Even systems that are able to add flexible network communication
calls create a dual access problem for their users (Fig. 5).
For example, consider a user programming a utility to read
and write magnetic tapes. If a tape drive is remote, it must
be accessed via the network communication system calls. On
the other hand, if the drive is local, it must be accessed
directly via a tape access operating system call. Since any
resource may be local or remote, users must always be prepared
to access each resource in two possible ways.

The problem of making local resources available to a network
has proven difficult for existing operating systems. The usual
approach is to have one or more "server" processes waiting for
requests from the network (Fig. 6). These server processes
then make local system calls to satisfy the remote requests
and return results through the network. Examples of this type
of server (though somewhat complicated by access control and
translation issues) are the ARPA network file transfer server
and Telnet user programs [6,
7]. With this approach there are
actually two service codes for each resource, one in the system
monitor for local service and one in the server process for
remote access.

The major network motivation for the New LTSS development is to
solve problems A. and B. in future versions of LTSS in such a
way as to avoid the dual access and dual service dichotomies.
By doing so, NLTSS also reaps some consequential benefits such
as user and server mobility, user extendibility, and others.

NLTSS provides only a single message system call (described in the next section).
Figure 7 illustrates the view that an NLTSS process has of
the world outside its memory space. Deciding how and where
to deliver message data is the responsibility of the NLTSS
message system and the rest of the distributed data delivery network.

Figure 7 - NLTSS processes have only the distributed message system
for dealing with the world outside their memory spaces.

There are two fundamentally opposite methods of avoiding the dual
access dichotomy: either make all resource accesses appear local,
or make all resource accesses appear remote. The TENEX Resource
Sharing EXECutive (RSEXEC) is an example of the former approach
[23].
Under the RSEXEC, system calls are trapped and interpreted to see if
they refer to local or remote resources. The interpreter must
then be capable of both access modes (Fig. 8).

NLTSS uses the opposite approach. Since all NLTSS resource requests
are made and serviced with message exchanges, the message system is
the only part of NLTSS that need distinguish between local and remote
transfers (Fig. 9). Also, since the distinction made by the message
system is independent of the message content, NLTSS eliminates the
dual access dichotomy rather than just moving it away from the user
as the RSEXEC and similar systems do.

NLTSS is able to avoid the dual service dichotomy by having the resource
service processes be the only codes that service resource requests
(Fig. 10). This means, however, that all "system calls" must go through
the NLTSS message system. The major difficulty faced in the NLTSS design
is to supply resource access with this pure message-passing mechanism
and yet still keep system overhead at least as low as that found in
the competing third generation operating
systems available to LLL.

There have been many operating system designs and implementations
that supply all resource access through a uniform interprocess
communication facility [1,
2, 3,
8, 10,
15, 16,
21, 24,
28].
These interprocess communication mechanisms generally do not extend
readily into a network, however. For example, in a system that
utilizes shared memory for communication, remote processes have
difficulty communicating with processes that expect such direct
memory access. Capability based systems generally experience
difficulty extending the capability passing mechanism into the
network [4,
8, 10,
16, 20,
21, 28].

NLTSS is certainly not the first pure message-passing system
[1, 15,
24].
In fact, it is remarkably similar to a system proposed by Walden
[24].
Any contributions that NLTSS has to make will come from the care
that was given to exclude system overhead and yet still support
full service access to local and remote resources through a
uniform message-passing mechanism.

Since all resource access in NLTSS is provided through the
message system, the message system interface is a key element
in the system design. The major goal of the NLTSS message system
interface design was to supply a simple, flexible communication
facility with an average local system overhead comparable to
the cost of copying the communicated data. To do this it was
necessary to minimize the number of times that the message
system must be called. Another important goal was to allow
data transfers from processes directly to and from local
peripherals without impacting the uniformity of the message
system interface.

The most important element in the NLTSS message system design
is a data structure that has been called a Buffer Table (Fig. 11).
A linked list of buffer tables is passed to the NLTSS message
system when a user process executes a system call (Fig. 12).

The NLTSS Buffer Table

Link

Action bits (Activate, Cancel, and Wait)

Send/Receive bit

Done bit

Beginning (BOM) and end (EOM) of message bits

Receive-From-any and Receive-To-Any bits

To and From network addresses

Base and length buffer description

Buffer offset pointer

Status

Figure 11 - The NLTSS Buffer Table.

Figure 12 - The NLTSS message system call.

The Buffer Table fields are used as follows:

The Link field is a pointer to the next Buffer Table (if any)
to be processed by the message system. When the message system is called,
it is passed the head of this linked list of Buffer Tables.
The linkage mechanism provides for data chaining of message pieces to and
from a single address pair, for activation of parallel data transfers,
and for waiting on completion of any number of data transfers.

The Action bits indicate what actions are to be performed by the message system during a call:

The Activate bit requests initiation of a transfer.
If the transfer can't be completed immediately because
the matching Buffer Table is remote or because of an insufficient
matching buffer size, the message system remembers the active Buffer Table for later action.

The Cancel bit requests deactivation of a previously activated
Buffer Table. The Cancel operation completes immediately
unless a peripheral is currently transferring into or out of the buffer.

The Wait action bit requests that the process be awakened when
this Buffer Table is Done (see Done bit below).

The Send/Receive bit indicates the direction of the data transfer.

The Done bit is set by the message system when a previously activated
Buffer Table is deactivated due to completion, error, or explicit Cancel.

The BOM and EOM bits provide a mechanism for logical message demarcation.
In a send Buffer Table, the BOM bit indicates that the first data bit in
the buffer marks the beginning of a message. Similarly, the EOM bit
indicates that the last bit in the buffer marks the end of a message.
For receive Buffer Tables the BOM and EOM bits are set to
indicate the separation in incoming data.

The Receive-From-Any and Receive-To-Any bits are only meaningful for
receive Buffer Tables. If on, they indicate that the Buffer Table will
match (for data transfer) a send Buffer Table with anything in the
corresponding address field (see below). Of course data will only be
routed to this receive buffer if it's "To" address actually addresses
the activating process. If an "Any" bit is set, the corresponding address
is filled in upon initiation of a transfer and the "Any" bit is turned off.

The To and From address fields indicate the address pair (or association)
over which the data transfer occurs. The From address is checked for validity.

The Offset field is updated to point just after the last bit of data in
the buffer successfully transferred (relative to Base).

The Status field is set by the message system to indicate the current state
of the transfer. It should be noted that the NLTSS message system
call is designed to minimize the number of times that a process must
execute a system call. Generally a process will call the message
system only when it has no processing left to do until some communication
completes. It is also important that messages of arbitrary length can
be exchanged (even by processes that have insufficient memory space to hold an entire message).

The BOM and EOM message separators are in many ways like virtual circuit opening
and closing indicators. It is expected that for NLTSS message systems
interfacing with virtual circuit networks (e.g. an X.25 network) that circuits
will be opened at the beginning of a message and closed at the end.
The first network protocol that the NLTSS message system will be interfaced
with, however, has been designed to eliminate the opening and closing of
circuits while still maintaining logical message separation very much as
the NLTSS message system interface does [13,
25, 26].

The paucity and simplicity of the NLTSS system calls allow its
monitor to be quite small and simple (a distinct advantage at
LLL where memory is always in short supply and security is an
important consideration).

Essentially all that is in the NLTSS monitor is the message call
handler and device drivers for directly attached hardware
devices (figure 4). In the case of the CPU device, the driver
contains the innermost parts of the scheduler (the so-called Alternator)
and memory manager (that is those parts that implement mechanism, not policy).

One property of the current NLTSS monitor implementations is that each
device driver must share some resident memory with a hardware interface
process for its device. For example, the storage driver must share some
memory with the storage access process, and the alternator must share
some memory with the process server. This situation is a little awkward
on machines that don't have memory mapping hardware.
On systems with only base and bounds memory protection, for example,
it forces the lowest level device interface processes to be resident.

The file system illustrates several features of the NLTSS design and implementations.

The basic service provided by the file system is to allow processes to read and
write data stored outside their memory spaces. The way in which a process gets
access to a file involves the NLTSS capability protocol
[26] and is beyond the
scope of this paper. We will assume that the file server has been properly
instructed to accept requests on a file from a specific network address.
The trust that the servers have in the "From" address delivered by the message
system is the basis for the higher-level NLTSS capability protection mechanisms
[10, 14].

The simplest approach for a file server to take might be to respond to a message
of the form "Read', portion description (To file server, From requesting process)
with a message containing either "OK”, data or "Error" (To requesting process, From file server).

Unfortunately, this approach would require that the file server be responsible
for both storage allocation (primarily a policy matter) and storage access (a mechanism).
Either that or the file server would have to flush all data transfers through
itself on their way to or from a separate storage access process.

The mechanism implemented in NLTSS is pictured in figure 13. To read or write a file,
a process must activate three Buffer Tables. For reading, it activates a send of
the command to the file server, a receive for the returned status, and a separate
receive for the data returned from the storage access process. For writing, it
activates similar command status Buffer Tables, but in place of a data receive,
it activates a data send to the storage access process.

Figure 13 - The NLTSS file system.

This example illustrates the importance of the linkage mechanism in the message
system interface. In most systems a file access request requires only one
system call. Through the linkage mechanism, NLTSS shares this property.
In fact, in NLTSS a process can initiate and/or wait on an arbitrary number of
other transfers at the same time. For example, when initiating a file request,
it may be desirable to also send an alarm request (return a message after T units of time)
and wait for either the file status message or the alarm response.

When the file server gets a read or write request, it translates the logical
file access request into one or more physical storage access requests that it
send to the storage access process. In this request it includes the network
address for the data transfer (this was included in the original "Read" or "Write" request).
Having received the storage access request, the access process can receive the
written data and write it to storage or read the data from storage and send it
to the "Read"ing process.

This mechanism works fine in the case where the requesting process and the
storage access process are on separate machines (note that the file server
can be on yet a third machine). In this case the data must be buffered as
it is transferred to or from storage. In the case where the requesting
process and the storage access processes are on the same machine, however,
it is possible to transfer the data directly to or from the memory space
of the requesting process. In fact, many third generation operating systems
perform this type of direct data transfer.

To be a competitive stand-alone operating system, NLTSS must also take advantage
of this direct transfer opportunity. In our implementations, the mechanism to
take advantage of direct I/O requires an addition to the message system.

There are two additional action bits available in the Buffer Tables of device
access processes, IOLock and IOUnLock. If a device access process wants to
attempt a direct data transfer, it sets the IOLock bit in its Buffer Table
before activation. If the message system finds a match in a local process,
instead of copying the data, it will lock the matching process in memory
and return the Base address (absolute), Length and Offset of its buffer
in the IOLocking Buffer Table. The device access process can then transfer
the data directly to or from storage. The IOUnLock operation releases the
lock on the requesting processes memory and updates the status of the
formerly locked Buffer Table.

The most important aspect of this direct I/0 mechanism is that it has no effect
on the operation of the requesting process OR on that of the file server.
Only the device access process (which already has to share resident memory
to interact with its device driver) and the message system need be aware of
the direct I/O mechanism.

The example of an NLTSS semaphore [9,
10] server can be used to
further illustrate the flexibility of the NLTSS message system.
The basic idea of the semaphore server is to implement a logical semaphore
resource to support the following operations:

"P": semaphore number (To semaphore server, From requester) - Decrement
the integer value of the semaphore by 1. If its new value is less than zero then
add the "From" address of the request to a list of pending notifications.
Otherwise send a notification immediately.

"V": semaphore number (To semaphore server, From requester) - Increment
the value of the semaphore by 1. If its value was less than zero then send a
notification to the oldest address in the pending notification list and remove
the address from the list.

Typically such a semaphore resource is used by several processes to coordinate
exclusive access to a shared resource (a file for example).
In this case, after the semaphore value is initialized to 1, each process
sends a "P" request to the semaphore server to lock the resource and
awaits notification before accessing it (note that the first such locking
process will get an immediate notification). After accessing the resource,
each process sends a "V" request to the semaphore server to unlock the resource.

An NLTSS implementation of such a server might keep the value of the semaphore
and a notification list for each supported semaphore.
The server would at all times keep a linked list of Buffer Tables used for
submission to the message system. This list would be initialized with some
number (chosen to optimize performance) of receive Buffer Tables "To"
the semaphore server and "From" any. These Buffer Tables would also have
their activate and wait action bits turned on.

The semaphore server need only call the message system after making a
complete scan of its receive Buffer Tables without finding any requests
to process (i.e. any with Done bits on). Any Done receive reque sts can
be processed as indicated above (l. and 2.). If a notification is to be
sent, an appropriate send Buffer Table with only the Activate action bit
on can be added to the Buffer Table list for the next message system call.
These send Buffer Tables are removed from the list after every message system call.

Processes may in general be waiting on some receive completions to supply more
data, and for some send completions to free up more output buffer space.
Even in this most general situation, however, they need only call the
message system when they have no processing left to do.

This semaphore server example can be compared with that given in
[10]
to illustrate how the network operating system philosophy has evolved
at LLL over the years. In earlier designs, for example, capabilities were
handled only by the resident monitor. In the NLTSS implementations,
the resident monitor handles only the communication and hardware multiplexing
described here. Resource access in NLTSS is still managed by capabilities,
but this matter is handled as a protocol between the users and servers
[26].
The integrity of the capability access protection mechanism is built on
the simpler data protection and address control implemented in the distributed
network message system of which NLTSS can be a component
[10, 14]

There are currently two versions of NLTSS running in an emulation mode,
one on a CDC 7600 and one on a Cray-1. These fledgling implementations
are being used to experiment with higher-level system protocols,
to develop and debug libraries, etc. The systems will be made completely
operational in his mode (except for device drivers) before being installed
as the resident monitor on any machines.

The NLTSS monitor and most of the servers are being written in a language
named Model [18,
19], a Pascal based language with data
type extension that
was developed at the Los Alamos Scientific Laboratory. Model generates
an intermediate language, U-Code (similar to Pascal's P-Code). We expect
this feature to help somewhat in moving NLTSS from machine to machine.

An important issue facing NLTSS is compatibility with existing software.
We expect little difficulty in supporting the type of requests available
from most of the library support routines at LLL. Reading and writing files,
terminal IIO, etc., pose no difficulty. The areas that cause the most
compatibility problems are those library routines that deal with very
system specific features of the existing LTSS systems.

For example, some existing software at LLL depends on a linear process
chain structure supported by the LTSS system. Even though the NLTSS
message system and capability-type process access protection are much
more general, we do plan to implement a fairly elaborate service facility
under NLTSS that mimics the linear LTSS process structure.
It is hoped that the use of this type of software will gradually lessen
as users become more familiar with the basic NLTSS Services.
In any case, since this mimicry is not part of the NLTSS monitor,
its use causes no more performance degradation than that caused by
running a brief additional user program.

Since NLTSS supplies all of its services through its message system,
processes on machines that can communicate with the NLTSS machine can
access NLTSS resources just as if they were local (except for performance).
Also, since NLTSS allows its processes to communicate with other machines
via the message system, any resources available on the network are available
to NLTSS processes.

Resource sharing is somewhat complicated by problems at both the very low
and very high end of the communication protocol scale. At the low end,
there is the problem of mapping the NLTSS message exchange into whatever
transport level protocol is available on the network (for example, what
do you do with the X.25 qualifier bit?). This problem is somewhat eased
at LLL by using an in-house protocol developed particularly to suit local network
applications [13,
25].

At the high end of the protocol scale, there is the problem of service
request-reply standards. The greatest difficulties involved in design
of message standards for a pure message-passing service are those
resulting from the domain restriction of the serving process(es).
Access control and accounting are examples of mechanisms that require
distributed coordination. Most third generation operating systems
assume that they control the entire computing facility.
This assumption is incorrect in a network like Octopus and creates
some serious problems. For example, resources serviced on one machine
can't be accessed from another, accounts may "run out" on one machine
and not on another, etc. Discussion of the distributed mechanisms
that NLTSS utilizes for services that require distributed control
is beyond the scope of this paper. Some of these mechanisms are described
in [26].
Additional details of the NLTSS message standards will be
described in later publications.

Implementation of a pure message-passing operating system
that efficiently utilizes the hardware resources available
to it is a considerable technical challenge. It is a challenge
that must be met, however, if the current software difficulties
involved in interconnecting operating systems to networks are
to be overcome. These software interconnection issues are
particularly pressing in a mature high performance local network
like the LLL Octopus network. It is hoped that the NLTSS development
effort will further the state of the art in software network
interconnections by giving birth to a viable message-passing
operating system in the demanding environment of the Octopus network.

W. Wulf, et. al.,"Hydra: Kernel of a Multiproprocessor Operating System,"
Communications of the ACM, Vol. 17, Number 6, June 1974.

DISCLAIMER

This work has been performed under the auspices of the
U.S. Department of Energy by the Lawrence Livermore
Laboratory under contract number W-7405-ENG-48.

About the author

James E. (Jed) Donnelley received bachelors degrees in Physics
and Mathematics (1970) and a masters degree in Mathematics
from the Davis campus of the University of California.
Since 1972 he has been a computer scientist at the Lawrence
Livermore Laboratory (LLL). Jed was technical liaison for
LLL's ARPA network node from 1973 to 1978
and has participated in research projects at LLL on
operating system security, distributed data bases,
local networks, and high performance computer architectures.
Since 1978 he has been primarily working on design and
implementation of a network operating system.
His principal research interests are in distributed computation,
cellular data flow computer architectures, and brain modeling.
Jed is a member of the ACM and the IEEE Computer Society.

Acknowledgements

The author wishes to acknowledge the assistance of his colleagues
on the NLTSS design and implementation teams: Pete DuBois,
Jim Minton, Chuck Athey, Bob Cralle and Dick Watson.
Particular thanks go to John Fletcher, who carried the day
in some early message system debates, and to Dick Watson,
whose continued support in the area of network protocols
has had a profound impact on NLTSS. This paper is a revised
version of a paper originally presented at the 4th Conference
on Local Networks, Minneapolis, Minnesota, 22-23 October, 1979.
The original paper was published in the proceedings of that
conference copyright 1979 IEEE. The permission of the IEEE to
utilize the original material is gratefully acknowledged.
Contact the author
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